This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. T lymphocytes are a major component of all vertebrate immune systems, necessary for clearance of virally infected cells and for stimulating B lymphocytes to make high affinity antibodies. While it has long been known that T cells form tight conjugates with cells when their antigen receptor detects particular antigens, the complex series of cell surface and cytoskeletal changes that accompany these changes are less well documented and understood. The resulting structure has been dubbed an "immunological synapse";it is characterized by a concentration of T cell receptor/MHC (major histocompatability molecules) in the center of the synapse and by intergrins and their ligands arranged in a peripheral ring. Underneath the synapse there is a massive rearrangement of the cytoskeleton that accompanies synapse formation. This involves actin, microtubules (including the MTOC), vesicles and signaling molecules. The architecture of this assembly is completely unknown. We have begun to study the cytoplasmic rearrangement that accompanies the formation of an immunological synapse using EM tomography of cells which have been preserved by rapid freezing and freeze substitution. Data from tomographic reconstructions of lymphocyte conjugates show an accumulation of both membranous and cytoplasmic components at the region of the synapse.